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Burr ragweed

family

Asteraceae

status

Burr ragweed (Ambrosia confertiflora) is listed as noxious in class 5 throughout NSW.

Description

Burr ragweed is an erect perennial herb 75-200cm high which grows in dense colonies arising off a system of rhizomes (underground runners). Stems are grey-green, erect, usually simple but branching in the flowering region. Leaves are grey-green, shortly stalked, opposite at the base of the plant, becoming alternate up the stems, deeply twice divided into long narrow segments, with a few stiff hairs on both surfaces. Individual flowers are small (to 4mm wide), yellow-green and grouped into terminal spikes. Seed is brown, 3-4mm across covered with 10-20 short hooked spines.

Preferred habitat and impacts

Introduced from southern USA and Mexico, burr ragweed is known from a few infestations in south-east Queensland and the western slopes and plains of NSW. Plants die back to the roots over winter but grow rapidly in spring and summer and flower in mid-summer. If dry spells occur in summer plants may die back and resprout in autumn.

Burr ragweed is not palatable to stock and by forming dense stands which exclude all other plants can reduce carrying capacity. The burrs cause vegetable fault in wool and are not easily removed because of the hooked hairs. It produces large amounts of allergenic pollen which can cause hay fever.

Dispersal

Seed is spread on animals or other items to which the seeds adhere, such as bags. The burrs are buoyant in water and may spread in floods. The plants spread locally by rhizomes. Movement of contaminated soil in roadworks can spread both seed and rhizome fragments.

Look-alikes

The plants are reminiscent of chrysanthemum, in the height and leaf form, but the flowers are inconspicuous. Several other weed species are similar, such as perennial ragweed (Ambrosia psilostachya) and Chinese mugwort (Artemisia verlotiorum).

Control

Cultivation is unlikely to be effective because of the extensive rhizome system. Herbicides would be the only possible means of controlling large infestations. Early detection and removal of seedlings is the best treatment in new areas of infestation.